(a) Field of the invention:
The present invention relates to a compact high vari-focal ratio zoom lens system to be attached to lens shutter cameras, etc.
(b) Description of the prior art:
In the recent years where cameras are increasingly fully automated, vari-focal lens systems are adopted in the photographic lens systems used in the so-called lens shutter cameras featuring portability. Under this circumstance, it is necessary to develop a compact zoom lens system.
Since a lens system to be attached to the lens shutter camera does not require a specific back focal length unlike a lens system for the single-lens reflex camera, the former lens system can be made compact by shortening its back focal length. For this purpose, it is preferable to compose a lens system, in the order from the object side, of a lens group having positive refractive power and a lens group having negative refractive power. There have already been designed, for example, compact quasi-wide angle single focal length lens systems and dual-focal lengths switching vari-focal lens systems.
As for zoom lens systems for use with the lens shutter cameras, there have been proposed the zoom lens system composed, in the order from the object side, of a lens group having positive refractive power and a lens group having negative refractive power, as well as the zoom lens system comprising three lens groups designed by developing the zoom lens system comprising two lens groups and the zoom lens system comprising four lens groups.
The zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 201213/57, for example, is composed, in the order from the object side of a first lens group having positive refractive power and a second lens group having negative refractive power.
Further, the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 137813/58 is so designed as to comprise three lens groups by dividing either of the first or second lens groups of the lens system composed of two lens groups into two lens groups.
The zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 57814/60 comprises four lens groups one of which is kept fixed during zooming together with a fixed stop.
The conventional zoom lens systems mentioned above are used with the lens shutter cameras, video cameras, etc., and designed for obtaining compact lens systems with no consideration to reserve definite back focal lengths unlike the case of the conventional lens systems for the single-lens reflex cameras.
However, all of the above-mentioned zoom lens systems have vari-focal ratios on the order of 1.5 to 2 and are insufficient for making use of perspective variations or obtaining the inherent in the telephoto lens system effect. Further, since the zoom lens systems are of the telephoto type having the lens composition of positive-negative refractive power distribution in the order from the object side, it is easy to shorten total lengths of the zoom lens systems. However, since the zoom lens systems have short back focal lengths at the wide position, some of the zoom lens systems have large lens diameters on the image side and are not compact.